A technique of forming a desired image by developing an electrostatic latent image formed on a photoconductor with a toner for developing electrostatic images, is widely used in image forming devices such as an electrophotography device, an electrostatic recording device and an electrostatic printing device. Such a technique is also applied to copying machines, printers, facsimiles and multi-functional printers thereof.
For example, generally, in an electrophotography device using an electrophotography method, an electrostatic latent image is formed on a photoconductor, which is made of a photoconductive material, by uniformly charging the surface of the photoconductor by various methods. Then, the electrostatic latent image is developed with a toner, transferred to a recording medium such as a paper, and then fixed by heating, etc., thereby obtaining a copy.
Toners used for development are divided into negatively-charged toners and positively-charged toners. Electrophotography devices which use positively-charged toners are preferably used in recent years because such devices produce less ozone and provide an excellent charge property.
In order to increase charge stability, flowability, durability, etc., of a toner, an external additive such as inorganic or organic fine particles generally having a smaller particle diameter than that of colored resin particles (toner particles) is added (attached) to the surface of toner particles used for development.
In conventional toners, the external additive is transferred to the inside of the toner particles from the surface of the toner particles and/or is released (detached) from the surface of the toner particles by a mechanical stress caused inside a developing device (an increase in the number of contacts between the toner particles by stirring or the like), etc., in the process of continuous printing of a large number of sheets. Therefore, there is a decrease in the function of the external additive and it becomes difficult to provide the toner particles with a stable charge property over time (charge stability).
The toner particles in which the external additive is embedded cause a phenomenon that the toner particles are attached to the surface of the photoconductor (filming phenomenon); therefore, a deterioration in image quality by fog or the like, is likely to occur. The external additive released (detached) from the surface of the toner particles deteriorates the charge property of the surface of the photoconductor, thereby causing a deterioration in printing durability of the toner and/or a deterioration in image quality. Especially under a condition of high temperature and high humidity, image quality is significantly deteriorated by filming.
Therefore, there is a demand for a toner which causes no failures such as the above-described embedding and/or releasing of the external additive, which can keep the external additive suitably attached over time, and which can maintain a stable charge property (charge stability) even in the case where, as in the case of continuous printing of a large number of sheets, there is an increase in the number of contacts between the toner particles inside the developing device by stirring or the like.
A toner is disclosed in Patent Literature 1, which uses inorganic/organic complex fine particles comprising organic fine particles and inorganic fine particles attached to the surface of the organic fine particles. Patent Literature 2 discloses a toner comprising, as the external additive, resin fine particles which have a specific average particle diameter and of which surface is covered with a continuous layer of silica.